This invention relates in general to frequency locking circuits and more particularly to an improved frequency locking circuit where frequency offsets caused by sideband imbalance are reduced.
In U.S. Pat. No. 4,631,498 to applicant, a wave meter/frequency locking technique is disclosed which is suitable for indirectly locking an optical frequency f to a radio frequency f' or for locking the radio frequency to the optical frequency. The indirect locking is accomplished by using a filter which has a characteristic frequency f.sub.f. A beam of optical frequency f is phase modulated by a subcarrier signal of average frequency f' that is itself modulated at frequency f". The modulated beam is passed through the filter to a detector to produce a detector output signal that has frequencies at linear integral sums of f' and f". A pair of control signals are generated that are proportional to the amplitude of the two components of the detector output signal. These control signals are used separately in a pair of servo loops to separately establish fixed values of f/f.sub.f and f'/f.sub.f. The value of f/f.sub.f is stepped to a different value. By measuring f'/f" at each of these values of f/f.sub.f, the value of f can be determined.
Imperfections in the phase or frequency modulation applied in the frequency locking circuit will cause a sideband asymmetry condition known as sideband imbalance. Such condition will cause a frequency offset in the device and is therefore undesirable. It is therefore desirable to provide frequency locking circuits in which frequency locking offsets caused by sideband imbalance are reduced.